Cloned murine leukemia virus (MuLV)-like human genomic DNA segments were sequenced and compared to Moloney (Mo) MuLV proviral DNA. The sequenced human DNA (about 2600 total nucleotides) was analyzed by computer for homology and location within the MoMuLV genome. Regions sequenced corresponded to portions of the gag proteins p12, p30, and p10, and reverse transcriptase (pol). Approximately 400 bases were also sequenced from a cloned fragment of African green monkey DNA which hybridized both to human and Mo-MuLV proviral DNA; sequence homology was also demonstrated. Another project dealt with two types of mouse MuLVs (ecotropic and xenotropic), which have unique host range specificity. Differences (hybridization) occur in the region encoding for envelope (env) glycoprotein. The sequence of portions of the pol and env regions of xenotropic proviral DNA was determined and compared to the published sequence of analogous segments of the AKR ecotropic provirus. Analyses showed nearly identical homology in the pol region, whereas short stretches of homology and regions of nucleotide deletions and insertions occurred in the env region. No major difference in the physical characteristics of the protein translated from xenotropic env-specific sequences was noted.